In modern dentistry, patients' demands not only for recovery of lost masticatory function, but also for recovery of dental aesthetics, are ever growing. Particularly in the case of the upper anterior area, the patient's aesthetic demands can be met when restorations are made such that the gingival topography matches the gingival contour. However, creating natural-looking soft tissue in the upper anterior area is a difficult task [1]. The criteria for successful soft tissue formation in the upper anterior area largely rely on the existence of a healthy interdental papilla and the formation of a matching gingival contour [2]. A deficiency in interdental papillae creates problems with pronunciation and food impaction, along with aesthetic issues [3]. Interdental papillae not only play a role in protecting the periodontal tissue in the upper anterior area by blocking food impaction, but also serve an aesthetic purpose [4].

To date, many studies have analyzed the factors influencing deficient interdental papillae. Among such studies, the study by Tarnow et al. [5] reported that interdental papillae were often present when the distance between the contact point and the bone crest (CP-BC) was ≤5 mm. Moreover, in a study by Cho et al. [6] on the effects of the interproximal distance between roots (IDR) on the presence of interdental papillae, according to the CP-BC, it was reported that, as IDR or CP-BC increased, the interdental papillae were less frequently present. In particular, when the CP-BC was 4–6 mm, an increase in the IDR resulted in a sensitive decrease in the presence of interdental papilla. Unlike the study by Tarnow et al. [5], which included both the anterior and posterior areas, Chen et al. [7] conducted a study on only the upper anterior area and reported that, when CP-BC was ≤4 mm, interdental papillae were present in 100% of the cases, whereas when this distance was ≥7 mm, interdental papillae were always deficient. Although other studies have reported that gender, interproximal embrasure area, interdental papilla length, and crown morphology are associated factors [8910], CP-BC is considered the most important associated factor for the presence of the interdental papilla [5678910].

Various periodontal plastic surgery methods have been suggested for the reconstruction of deficient interdental papillae. Although a variety of surgical procedures and flap designs have been suggested [1112131415], they have had limited success and have lacked predictability; as such, interdental papilla reconstruction is considered one of the most challenging periodontal plastic surgery procedures [15]. Therefore, aesthetic enhancement of deficient interdental papilla requires the use of aesthetic restorative resin, prosthetic restoration, or an orthodontic approach. However, Becker et al. [16] used commercial injectable hyaluronic acid gel for deficient interdental papillae in a small area, the first report to show that minimally invasive and predictable reconstruction of deficient interdental papillae was possible. Hyaluronic acid is a member of the glycosaminoglycan family and a major component of the extracellular matrix in almost all tissues. Its primary role is to bind water to maintain tissue structure and its characteristics, including consistency, biocompatibility, and hydrophilicity, have made it an excellent moisturizer in cosmetic dermatology and skin-care products. To date, injectable hyaluronic acid gel has been successfully used to reduce wrinkles and improve other similar facial deformities [17].

Accordingly, the present study aimed to investigate the effects of hyaluronic acid gel injection for the reconstruction of deficient interdental papillae in the upper anterior area, using an image analysis system to assess the clinical and radiographic factors influencing reconstruction of the interdental papilla by means of this procedure.

After performing interdental papilla reconstruction using injectable hyaluronic acid gel on a total of 13 patients at 57 interdental papilla sites in the upper anterior area, a mean IPRR of 88.80% was found with 3.33 injections, while BTA, BTH, and BTW showed a mean decrease of 0.21 mm², 0.70 mm, and 0.30 mm, respectively (Table 1).

CIPR was found in 36 treated sites, while the remaining 21 sites showed an IPRR ranging from 19% to 96%. With respect to comparison of the mean values of clinical and radiographic variables between the CIPR (Figure 4) and PIPR (Figure 5) groups, the initial BTA, BTH, BTW, and CP-BC showed statistically significant differences, whereas the IDR did not show a statistically significant difference between these groups (Table 2).

As the CP-BC increased, the initial BTA, BTH, and BTW also increased, and were strongly positively correlated (Figure 6A), while the correlation with IDR weakened (Figure 6B).

Although the CP-BC showed a statistically significantly strong correlation with the IPRR, there was no statistically significant correlation between the IDR and IPRR (Table 3). For CP-BC up to 6 mm, an IPRR of essentially 100% was observed, while for a value >6 mm, an increase in distance showed a statistically significantly negative correlation with the IPRR (Figure 7).

Some studies have already reported that successful interdental papilla reconstruction is possible by using commercial injectable hyaluronic acid gel. In a study by Becker et al. [16], it was possible to achieve 100% CIPR at 3 of 14 sites with a maximum of 3 injections of injectable hyaluronic acid gel at 3-week intervals, while the remaining 11 sites showed an IPRR of 57%–97%. Mansouri et al. [20] used the same experimental methods as Becker et al. [16] and reported a wide-ranging IPRR of 22%–100% at a 6-month follow-up of a total of 21 sites in the upper anterior area, with 9 sites showing an IPRR of ≥50%. In the present study, 52 of a total of 57 sites showed an IPRR of ≥50%. Moreover, CIPR occurred at 36 sites, which is a higher percentage than that reported in other studies, while the remaining 21 sites showed wide-ranging partial reconstruction (19%–96%). These differences may be attributable to differences in the initial interdental papillary deficiency levels and the experimental design. The studies by Becker et al. [16] and Mansouri et al. [20] did not provide information on the initial interdental papillary deficiency levels, making it difficult to make a direct comparison with the results from the present study; however, it is possible that areas with less initial interdental papillary deficiency may have been included in the present study. Moreover, the effect of interdental papilla reconstruction may have been enhanced by more precise injections, made possible by using an injection-assistance device and a 30G needle, as well as the fact that the number of injections was increased to 5, at 3-week intervals.

When the mean values of each variable were compared between the CIPR and PIPR groups, the initial interdental papillary deficiency level and the CP-BC were different to a statistically significant degree. Previous studies [5678910] have reported that the CP-BC was the most important factor in interdental papillary deficiency. Moreover, considering that the deficient area of the interdental papilla also increased as the CP-BC increased in the present study (Figure 6A), it can be surmised that the CP-BC is also an important factor for CIPR using injectable hyaluronic acid gel. However, the present study did not find any association between the initial interdental papillary deficiency level and the IDR (Figure 6B). Earlier studies [69] reported that the presence of the interdental papilla decreased with an increasing IDR, but recent studies [710] have reported that when the CP-BC variable was controlled, the influence of the IDR on the presence of the interdental papilla disappeared. Thus, the IDR has a minimal influence on interdental papilla reconstruction using injectable hyaluronic acid gel.

Interestingly, most cases with a CP-BC of up to 6 mm showed CIPR, but as the distance increased beyond this cut-off, the IPRR tended to decrease (Figure 7). Interdental papillae are generally considered biologically safe and stable with a CP-BC up to 5 mm [7]. Tarnow et al. [5] have also reported that there was virtually no interdental papillary deficiency at a CP-BC ≤5 mm. Injectable hyaluronic acid gel has a predictable interdental papilla reconstruction effect of approximately 1 mm in the vertical direction. Therefore, a CP-BC value of 6 mm can be used as a cut-off value for interdental papilla reconstruction using hyaluronic acid gel.

In the present study, we used a photographic standardization device developed in-house to increase the reproducibility of clinical photographs taken of the interdental papilla areas. In studies by Becker et al. [16] and Mansouri et al. [20], initial photographs were taken perpendicular to teeth of interest and these were used to take subsequent photos as close to the original photos as possible without a special device. Such a method required the utmost effort to reproduce the same horizontal and vertical angles during photography, but we tried to minimize errors in future imaging analysis measurements of the black triangle area by making sure that the brown strip on the cylindrical post was not visible on each clinical photograph acquired. However, although the reproducibility of the photos was improved compared to previous studies, it was still difficult to reproduce a photo at the same corresponding angle (Figures 4 and 5). Therefore, more studies are needed in the development of a more precise reproducible alignment device and method.

Within the limits of this study, the CP-BC was found to be correlated with the BTA, BTH, and BTW, but not with the IDR. Interdental papilla reconstruction using injectable hyaluronic acid gel allowed successful reconstruction when the CP-BC was ≤6 mm, but for the CP-BC >6 mm, the increased distance resulted in a decreased IPRR. Therefore, the CP-BC is closely associated with the efficacy of hyaluronic acid gel injection for the reconstruction of deficient interdental papillae in the upper anterior area.